Various sensors are known in the pressure sensing arts. In particular, many different techniques have been proposed for sensing the pressure in tires and for delivering this information to the operator at a central location on the vehicle so that he knows that a tire is at low or high air pressure.
Such pressure sensors generally communicate with the vehicle so that the sensed pressure is displayed to the operator when the vehicle is moving, i.e. the wheel rotating relative to the body of the vehicle. Such devices are generally relatively complex and expensive or alternatively are not particularly robust.
Some tire pressure sensor systems incorporate a sensor that is fixed to the body so no rotating electrical contact between the rotating wheel and the chassis is required. In this system, a sensor rod is deflected by contact with the tire sidewall when the sidewall of the tire is deformed as occurs when the tire pressure is low. This system provides an indication of low tire pressure but is not robust. For example mud or other debris on the wheels may cause faulty readings. Furthermore, this system provides an indication only when the tire pressure is reduced significantly as is necessary for significant tire bulge to occur. Clearly such a system simply cannot provide a reading of actual tire pressure.
In another form of fixed sensor the height of the vehicle can be detected and when the height is reduced, it is deemed tire pressure is low. However, if the tire in a rut or is parked on uneven ground, a faulty low-pressure reading is likely to be generated.
More complicated systems are capable of monitoring tire pressure. For example, some pressure sensor systems utilize a rotating encoder formed by a multipolar ring of magnetic segments of different polarity that are distributed circumferentially in a regular and alternating manner. A transmitter coil coaxial with the ring and a fixed pickup (an induction coil system) is energized by alternating electrical current flowing through the transmitter coil to generate a magnetic field superimposed on the magnetic field created by the multipolar ring generates a signal picked up and delivers a signal relating the rotating characteristic of the wheel and thus, the state of the tire.
Some tire pressure systems also utilize a wheel system wherein each sensor on each wheel is provided with a radio transmitter that transmit the information on tire pressure, etc. from the wheel to a radio receiver on the body of the vehicle and this transmitted signal is decoded to provide information on tire pressure etc. and makes it available to the operator. Conventional wireless systems, however, are not durable and are expensive to design and produce.
SAW pressure sensors, for example, can be composed of a sense element on a base and pressure transducer sensor diaphragm that is part of the cover. For the SAW sensor to function properly, the sensor diaphragm must be in intimate contact with the sense element at all pressure levels and temperatures. To compensate for expansion in the packaging, the sense element and sensor diaphragm must be preloaded when they are assembled to shift the output frequency a known amount, which ensures contact at all times.
In conventional sensor designs, an interference fit between the cover and base can maintain a preload until the cover and base are locked in place by welding, soldering or other connecting means. This type of interference fit can weaken before the parts are locked together and cause the preload to be reduced.
The present inventors have therefore concluded that a need exists for an improved sensor design that reduces or eliminates altogether the need for an interference fit, as indicated above.